Process for producing nanoparticles

ABSTRACT

A process comprises (a) combining (1) at least one base and (2) at least one metal carboxylate salt comprising (i) a metal cation selected from metal cations that form amphoteric metal oxides or oxyhydroxides and (ii) a carboxylate anion comprising from one to four alkyleneoxy moieties, or metal carboxylate salt precursors comprising (i) at least one metal salt comprising the metal cation and a non-interfering anion and (ii) at least one carboxylic acid comprising from one to four alkyleneoxy moieties, at least one salt of the carboxylic acid and a non-interfering, non-metal cation, or a mixture thereof; and (b) allowing the base and the metal carboxylate salt or metal carboxylate salt precursors to react.

STATEMENT OF PRIORITY

This application claims the priority of U.S. Provisional Application No.60/871,326 filed Dec. 21, 2006, the contents of which are herebyincorporated by reference.

FIELD

This invention relates to processes for producing metal oxide or metaloxyhydroxide particles.

BACKGROUND

Metal oxides have numerous uses. In particular, zinc oxide is used fordiverse purposes including, for example, use as a white pigment, as acatalyst, as a constituent of antibacterial skin-protection ointment,and as an activator for rubber vulcanization. Sunscreens and woodvarnishes contain finely divided zinc oxide as an ultraviolet(UV)-absorbing pigment.

Zinc oxide is useful as a UV-absorbing agent because it does not degradeupon prolonged exposure to UV light. When its particle size is less thanabout 20 nanometers (nm), however, its bandgap shifts to higher energyas its particle size decreases, due to quantum confinement. To maximizethe number of UV wavelengths absorbed by zinc oxide, particles having abandgap as close as possible to the semiconductor's bulk bandgap aredesirable. Since the shift from the bandgap of the bulk material isgreater the smaller the particle size, crystalline particle diameters ofat least about 5 nm generally can be useful. Such particle diametersprovide bandgap values relatively close to those of the bulk material,resulting in a relatively broad range of absorbed wavelengths.

Nanoparticles of zinc oxide can be sufficiently small, however, so as toscatter only negligible amounts of visible light. Thus, UV lightabsorbing, but visible light transparent, composites (for example,transparent organic-inorganic hybrid materials, plastics, paints andcoatings) can be made using zinc oxide nanoparticles as a filler. Tomaintain optical transparency, particle diameters (and the diameters ofany agglomerates present) generally should be less than about one-tenththe wavelength of light (for example, below about 30 nm).

The preparation of zinc oxide by both dry and wet processes is known.The classical dry method of burning zinc generates aggregated particleshaving a broad size distribution. Particularly finely divided zinc oxideis prepared predominantly by wet chemical methods using precipitationprocesses. Precipitation in aqueous solution generally gives hydroxide-and/or carbonate-containing materials that require thermal conversion tozinc oxide. The thermal post-treatment can have a negative effect on thefinely divided nature of the particles, as the particles are subjectedduring this treatment to sintering processes that can lead to theformation of micrometer (μm)-sized aggregates. These aggregates can bebroken down only incompletely to the primary particles by milling orgrinding.

In non-aqueous solutions (or aqueous solutions above the decompositiontemperature of zinc hydroxide), zinc oxide can be grown through a simplebase precipitation according to the following equation (where X isgenerally a suitable anion and Y is a suitable cation):

ZnX₂+2YOH→ZnO⁺²YX+H₂O

Particle growth takes place through an Ostwald ripening process and isdiffusion-dependent. As such, particle growth is rather slow at roomtemperature if 8 nm or larger diameter particles are desired. Elevatingthe reaction temperature can speed the process to reasonable rates, butthis can simultaneously increase the rate of agglomeration.

Various common zinc salts (for example, zinc acetate) have been used asthe starting salt in such non-aqueous precipitation processes. However,such starting salts have generally required the use of dilute solutionsto avoid relatively high rates of agglomeration, and zinc oxide grownfrom such salts has tended to form agglomerates that are unsuitable forapplications requiring transparency.

Other processes for the preparation of nanosize zinc oxide particlesutilize expensive starting materials (for example, zinc alkoxides),require the use of emulsifiers, are complex, provide agglomerates,provide slow particle growth, provide insufficient control over particlesize, and/or cannot provide often preferred particle sizes (for example,average primary particle diameters of about 5 to about 30 nm).

SUMMARY

Thus, we recognize that there is a need for processes for producingmetal oxide or metal oxyhydroxide nanoparticles (particularly, zincoxide nanoparticles) that can minimize or even eliminate particleagglomeration, while allowing for particle growth to desired primaryparticle sizes. Preferred processes will be simple, cost-effective,and/or enable control of final particle size.

Briefly, in one aspect, this invention provides such a process, whichcomprises (a) combining (preferably, in at least one solvent) (1) atleast one base and (2) at least one metal carboxylate salt comprising(i) a metal cation selected from metal cations that form amphotericmetal oxides or oxyhydroxides (most preferably, zinc) and (ii) acarboxylate anion comprising from one to four alkyleneoxy moieties, or,alternatively, metal carboxylate salt precursors comprising (i) at leastone metal salt comprising the metal cation and a non-interfering anion(that is, an anion that is not reactive with the base) and (ii) at leastone carboxylic acid comprising from one to four alkyleneoxy moieties, atleast one salt of the carboxylic acid and a non-interfering, non-metalcation (for example, tetraalkylammonium; preferably,tetramethylammonium), or a mixture thereof; and (b) allowing the baseand the metal carboxylate salt or metal carboxylate salt precursors toreact (for example, to form a metal oxide or metal oxyhydroxide).Preferably, the carboxylate anion and carboxylic acid further comprise aterminal group selected from alkyl, hydroxyl, and the like (morepreferably, alkyl; most preferably, methyl) and/or the alkyleneoxymoieties comprise ethyleneoxy and/or propyleneoxy moieties (morepreferably, ethyleneoxy moieties).

It has been discovered that use of the above-described metal carboxylatesalts or metal carboxylate salt precursors in a basic precipitationprocess can enable the preparation of substantially non-agglomeratedmetal oxide or metal oxyhydroxide nanoparticles. In addition, the saltscan enable nanoparticles to be grown to preferred larger average primaryparticle sizes (for example, average primary particle diameters above4-5 nm). Preferred embodiments of the process can enable control ofaverage primary particle size by varying, for example, the reactiontemperature and/or time.

Thus, the process of the invention can be especially advantageous forproducing zinc oxide nanoparticles. The process can be used to provide,for example, zinc oxide nanoparticles having average primary particlediameters in the range of about 5 nm to about 10 nm or more. Suchnanoparticles can be well-suited for use in making UV light absorbing,visible light transparent composites, with the particle size controlthat is provided by the process further enabling a tuning of absorptioncharacteristics.

The process of the invention, in addition, is relatively simple andutilizes metal carboxylate salts or metal carboxylate salt precursorsthat are relatively inexpensive starting compounds. Thus, in at leastpreferred embodiments, the process can meet the above-mentioned need inthe art for simple, cost-effective processes for producing metal oxideor metal oxyhydroxide nanoparticles (particularly, zinc oxidenanoparticles) that can minimize particle agglomeration, while allowingfor particle growth to desired primary particle sizes.

BRIEF DESCRIPTION OF DRAWING

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawing, wherein:

FIG. 1 is a plot of average primary particle diameter versus time forthe process embodiments described in Comparative Example 1(a) (usingonly zinc acetate, Zn(Ac)₂) and Examples 1, 2, 3, and 4 (using varyingratios of Zn(Ac)₂ to zinc 2-(2-methoxyethoxy)acetate, Zn(MEAc)₂).

FIG. 2 is a plot of the log of average agglomerated particle diameterversus time for the process embodiments described in Comparative Example1(a) (using only zinc acetate, Zn(Ac)₂) and Examples 1, 2, 3, and 4(using varying ratios of Zn(Ac)₂ to zinc 2-(2-methoxyethoxy)acetate,Zn(MEAc)₂).

FIG. 3 is a plot of average primary particle diameter versus time forthe process embodiments described in Comparative Example 1(b) (usingonly zinc acetate, Zn(Ac)₂) and Examples 5, 6, 7, and 8 (using varyingratios of Zn(Ac)₂ to zinc 2-[2-(2-methoxyethoxy)ethoxy]acetate,Zn(MEEAc)₂).

FIG. 4 is a plot of the log of average agglomerated particle diameterversus time for the process embodiments described in Comparative Example1(b) (using only zinc acetate, Zn(Ac)₂) and Examples 5, 6, 7, and 8(using varying ratios of Zn(Ac)₂ to zinc2-[2-(2-methoxyethoxy)ethoxy]acetate, Zn(MEEAc)₂).

FIG. 5 is a plot of average primary particle diameter versus time forthe process embodiments described in Comparative Example 2 (using onlyzinc acetate, Zn(Ac)₂) and Examples 9, 10, 11, and 12 (using varyingratios of Zn(Ac)₂ to zinc methoxyacetate, Zn(MAc)₂).

FIG. 6 is a plot of the log of average agglomerated particle diameterversus time for the process embodiments described in Comparative Example2 (using only zinc acetate, Zn(Ac)₂) and Examples 9, 10, 11, and 12(using varying ratios of Zn(Ac)₂ to zinc methoxyacetate, Zn(MAc)₂).

FIG. 7 is a plot of average primary particle diameter versus time, aswell as a plot of the log of average agglomerated particle diameterversus time, for the process embodiment described in Example 13 (usingzinc 2-methoxy-1-methylethoxy acetate, Zn(MMEAc)₂).

FIG. 8 is a plot of average primary particle diameter versus time forthe process embodiments described in Comparative Example 3 (using onlyzinc acetate, Zn(Ac)₂) and Example 14 (using zinc2-methoxy-1-methylethoxy acetate, Zn(MMEAc)₂).

FIG. 9 is a plot of the log of average agglomerated particle diameterversus time for the process embodiments described in Comparative Example3 (using only zinc acetate, Zn(Ac)₂) and Example 14 (using zinc2-methoxy-1-methylethoxy acetate, Zn(MMEAc)₂).

DETAILED DESCRIPTION Definitions

As used in this patent application:

“agglomeration” means an association of primary particles, which canrange from relatively weak (based upon, for example, charge or polarity)to relatively strong (based upon, for example, chemical bonding);

“amphoteric” (in reference to a metal oxide or metal oxyhydroxide) meansable to function as both a Bronsted/Lowry acid and base;

“nanoparticles” means particles having a diameter of less than about 100nm;

“oligomeric” (in regard to an alkyleneoxy segment) means having fewerthan 5 alkyleneoxy moieties (which can be the same or different);

“primary particle size or diameter” means the size or diameter of anon-associated single crystal particle; and

“sol” means a dispersion or suspension of colloidal particles in aliquid phase.

Bases

Bases that can be suitable for use in the process of the inventioninclude hydroxyl group-containing basic compounds and mixtures thereof.Useful compounds include sodium hydroxide, potassium hydroxide, ammoniumhydroxide, tetramethylammonium hydroxide, and the like, and mixturesthereof. Preferred bases include sodium hydroxide (for example, due toits relatively low cost), tetramethylammonium hydroxide (for example,due to its solubility in a wide variety of organic solvents), andmixtures thereof. Tetramethylammonium hydroxide is more preferred.

The base can be used in solid form (for example, as NaOH or KOH pellets)or in the form of a solution in a polar organic solvent (for example, analkanol such as methanol). A wide range of concentrations can be useful(for example, tetramethylammonium hydroxide can be used in acommercially available concentration of 25 weight percent in methanol).In a preferred embodiment of the process of the invention, the base canbe added in solution form to a solution of metal carboxylate salt ormetal carboxylate salt precursors. Solvents useful for dissolving thebase include acetone, diethylether, alkanols (for example, methanol,ethanol, and isopropanol), dimethylsulfoxide (DMSO), dimethylformamide(DMF), tetrahydrofuran (THF), ethyl acetate, and the like, and mixturesthereof, with alkanols being preferred and methanol more preferred.

Salts

Metal carboxylate salts suitable for use in the process of the inventioninclude those that comprise (i) a metal cation selected from metalcations that form amphoteric metal oxides or oxyhydroxides and (ii) acarboxylate anion comprising from one to four alkyleneoxy moieties.Suitable metals include Be, Ti, V, Mn, Cr, Fe, Co, Ni, Al, Zn, Ga, In,Ge, Sn, Pb, As, Sb, Bi, Te, Po, and the like, and mixtures thereof.Preferred cations of such metals include Be⁺², Ti⁺⁴, V⁺⁴, V⁺⁵, Mn⁺⁴,Cr⁺³, Cr⁺⁴, Fe⁺³, Fe⁺⁴, Co⁺³/Co⁺² (mixed oxidation state compound),Ni⁺³, Ni⁺⁴, Al⁺³, Zn⁺², Ga⁺³, In⁺³, Ge⁺², Sn⁺², Sn⁺⁴, Pb⁺², As⁺³, Sb⁺³,Bi⁺³, Te⁺⁴, Po⁺⁴, and the like, and mixtures thereof. Preferred metalsinclude Ti, V, Mn, Cr, Al, Zn, Ga, In, Sn, Pb, and mixtures thereof.More preferred are Ti, Al, Zn, Ga, In, and mixtures thereof, with Znbeing most preferred. If desired, the salts can comprise other metalcations (non-amphoteric) (for example, at levels up to about 10 molepercent, based upon the total number of moles of metal cation), butpreferably all metals in the salt are selected from those that formamphoteric metal oxides or oxyhydroxides.

Preferably, the carboxylate anion further comprises a terminal groupselected from alkyl, hydroxyl, and the like. More preferably, theterminal group is an alkyl group, which can be linear or branched(preferably, linear) and preferably has from one to about four carbonatoms. The alkyl group is preferably methyl, ethyl, or isopropyl (morepreferably, methyl or ethyl; most preferably, methyl).

The alkyleneoxy moieties form an oligomeric alkyleneoxy segment, whichcan further comprise other moieties, and which, when present in thecarboxylate anion, preferably has only two or three alkyleneoxymoieties. Suitable alkyleneoxy moieties can be linear or branched andsubstituted or unsubstituted (preferably, unsubstituted). Thesubstituents can be organic substituents (that is, hydrocarbyl orheteroatom-containing hydrocarbyl). Suitable alkyleneoxy moietiesinclude those having from 1 to about 4 carbon atoms and, whensubstituted, can comprise up to a total of about 7 non-hydrogen atoms.Thus, useful alkyleneoxy moieties include methyleneoxy, ethyleneoxy,n-propyleneoxy, i-propyleneoxy, butyleneoxy, and the correspondingmethoxy- and ethoxy-substituted moieties, and mixtures thereof (as themoieties within the segment can be the same or different). Preferably,the alkyleneoxy moieties comprise ethyleneoxy and/or propyleneoxymoieties (more preferably, ethyleneoxy moieties; most preferably, one ortwo ethyleneoxy moieties).

A class of useful metal carboxylates can be represented by the followinggeneral formula

[R′(OR)_(x)Z(OR)_(w)(CH₂)_(y)COO⁻]_(m)[X⁻]_(n)M^(+(m+n))  (I)

wherein R′ is a linear or branched alkyl group having from 1 to about 4carbon atoms; each R is independently a linear or branched alkylenemoiety having from 1 to about 4 carbon atoms; x is an integer of 0 to 4;Z is a divalent organic linking moiety (for example, a moietynon-directionally selected from the group consisting of a covalent bond,—S—, —C(O)O—, —C═C—, and —C(O)NH—, and combinations thereof); w is aninteger of 0 to 4, with the proviso that the sum of x+w is an integer of1 to 4; y is an integer of 0 to about 3; X is a non-interfering anion(that is, an anion that is not reactive with base); m and n are integershaving values such that the sum m+n is equal to the charge of the metalcation, M; and at least about 90 mole percent (preferably, at leastabout 95 mole percent; more preferably, about 100 mole percent) of M(based upon the total number of moles of metal cation) is selected fromBe, Ti, V, Mn, Cr, Fe, Co, Ni, Al, Zn, Ga, In, Ge, Sn, Pb, As, Sb, Bi,Te, Po, and mixtures thereof.

Preferably, R′ is methyl, ethyl, or isopropyl (more preferably, methylor ethyl; most preferably, methyl); each R is independently a linear orbranched alkylene moiety having from 1 to about 3 carbon atoms (morepreferably, 1 or 2); x is an integer of 1 to 3 (more preferably, 2 or 3;most preferably, 2); Z is selected from the group consisting of acovalent bond, —C(O)O—, —C═C—, —C(O)NH—, and combinations thereof (morepreferably, a covalent bond, —C═C—, —C(O)O—, and combinations thereof,most preferably, a covalent bond); w is an integer of 0 to 1 (mostpreferably, w is an integer of 0), with the proviso that the sum of x+wis an integer of 1 to 4; y is an integer of 0 to about 2 (mostpreferably, 0); X is an anion selected from halide, nitrate, acetate,carbonate, formate, propionate, sulfate, bromate, perchlorate,tribromoacetate, trichloroacetate, trifluoroacetate, and chlorate ions,and mixtures thereof (more preferably, selected from chloride, acetate,and mixtures thereof, most preferably, acetate); and/or M is selectedfrom Ti, V, Mn, Cr, Al, Zn, Ga, In, Sn, Pb, and mixtures thereof (morepreferably, Ti, Al, Zn, Ga, In, and mixtures thereof, most preferably,zinc).

Representative examples of useful metal carboxylates include zinc2-[2-(2-methoxyethoxy)ethoxy]acetate, zinc 2-(2-methoxyethoxy)acetate,zinc methoxyacetate, zinc succinate mono-(2-methoxyethyl) ester, zincsuccinate mono-[2-(2-methoxyethoxy)ethyl]ester, zinc succinatemono-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}ester, zinc succinatemono-(2-methoxy-1-methylethyl) ester, zinc succinatemono-[2-(2-methoxy-1-methylethoxy)-1-methylethyl]ester, zinc succinatemono-{2-[2-(2-methoxy-1-methylethoxy)-1-methylethoxy]-1-methylethyl}ester,zinc [3-(2-methoxyethoxy)propoxy]acetate, zinc(2-methoxyethoxymethoxy)acetate, zinc succinatemono-{2-[2-(2-methoxyethoxy)-1-methylethoxy]-1-methylethyl}ester, zinc3-[2-(2-methoxyethoxy)ethoxy]propionate, zinc[2-(2-methoxyethoxy)-1-methylethoxy]acetate, zinc pentanedioatemono-[2-(2-methoxyethoxy)ethyl]ester, zinc pentanedioatemono-(2-methoxy-1-methylethyl) ester, zinc(2-methoxyethoxycarbonylmethoxy)acetate, zinc(2-methoxy-1-methylethoxycarbonylmethoxy)acetate, zinc but-2-enedioatemono-(2-methoxyethyl) ester, zinc N-(2-methoxyethyl)succinamate, zinc[(2-methoxyethylcarbamoyl)methoxy]acetate, zinc[2-(2-ethoxyethoxy)ethoxy]acetate, zinc[2-(2-isopropoxyethoxy)ethoxy]acetate, zinc{2-[2-(2-tert-butoxyethoxy)ethoxy]ethoxy}acetate, zinc{2-[2-(2-sec-butoxyethoxy)ethoxy]ethoxy}acetate, zinc2,3-dimethoxypropionate, zinc succinate mono-(2,3-dimethoxypropyl)ester, zinc succinate mono-(2-methoxy-1-methoxymethylethyl) ester, zinc3-(2-methoxyethylcarbamoyl)acetate, zinc (2-hydroxyethoxy)acetate, zinc[2-(2-hydroxyethoxy)ethoxy]acetate, zinc3-{2-[2-(2-methoxy-1-methylethoxy)-1-methylethoxy]-1-methylethoxy}propionate,zinc 3-(2-methoxy-1-methylethoxy)propionate, zinc{2-[2-(2-methoxy-1-methylethoxy)-1-methylethoxy]-1-methylethoxy}acetate,zinc (2-methoxy-1-methylethoxy)acetate, and the like, and mixturesthereof.

Preferred metal carboxylates include all of the above-listedrepresentative metal carboxylates that comprise a terminal methyl groupand mixtures thereof. More preferred metal carboxylates include2-[2-(2-methoxyethoxy)ethoxy]acetate, zinc 2-(2-methoxyethoxy)acetate,zinc methoxyacetate, zinc succinate mono-(2-methoxyethyl) ester, zincsuccinate mono-[2-(2-methoxyethoxy)ethyl]ester, zinc succinatemono-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}ester, zinc succinatemono-(2-methoxy-1-methylethyl) ester, zinc succinatemono-[2-(2-methoxy-1-methylethoxy)-1-methylethyl]ester, zinc succinatemono-{2-[2-(2-methoxy-1-methylethoxy)-1-methylethoxy]-1-methylethyl}ester,zinc3-{2-[2-(2-methoxy-1-methylethoxy)-1-methylethoxy]-1-methylethoxy}propionate,zinc 3-(2-methoxy-1-methylethoxy)propionate, zinc{2-[2-(2-methoxy-1-methylethoxy)-1-methylethoxy]-1-methylethoxy}acetate,zinc (2-methoxy-1-methylethoxy)acetate, and mixtures thereof.

Zinc 2-[2-(2-methoxyethoxy)ethoxy]acetate (which can be represented asZn(MEEAc)₂), zinc 2-(2-methoxyethoxy)acetate (which can be representedas Zn(MEAc)₂), zinc methoxyacetate (which can be represented byZn(MAc)₂), zinc (2-methoxy-1-methylethoxy)acetate (which can berepresented by Zn(MMEAc)₂), and mixtures thereof are most preferred.

Such metal carboxylates can be prepared from the corresponding metalsalts having anions that can be displaced with carboxylic acids. Usefulstarting metal salts include metal oxynitrates, metal oxychlorides,metal carbonates, metal acetates, metal formates, metal propionates,metal nitrates, metal chlorides, metal oxides, metal hydroxides, metaloxyhydroxides, and the like, and combinations thereof. Many of suchsalts are commercially available.

Metal carboxylates can be obtained from the reaction of such startingmetal salts with polyether carboxylic acids. Suitable polyethercarboxylic acids include water soluble monocarboxylic acids (that is,containing one carboxylic acid group per molecule) having a polyethertail. The polyether tail can comprise repeating divalent alkyleneoxygroups (—O—R—). Preferred R groups can have the general formula—C_(n)H_(2n)— and include, for example, methylene, ethylene, andpropylene (including n-propylene and i-propylene), and combinationsthereof.

A class of useful polyether carboxylic acids can be represented by thegeneral formula

R′(OR)_(x)Z(OR)_(w)(CH₂)_(y)COOH  (II)

where R′, R, x, Z, w, and y are as defined above for Formula (I).

Representative examples of useful polyether carboxylic acids includethose that correspond to the above-listed representative metalcarboxylates, and the like, and mixtures thereof. Most preferredpolyether carboxylic acids include 2-[2-(2-methoxyethoxy)ethoxy]aceticacid having the chemical structure CH₃—O—(CH₂CH₂O)₂CH₂COOH (hereinafter,MEEAA), 2-(2-methoxyethoxy)acetic acid having the chemical structureCH₃OCH₂CH₂OCH₂COOH (hereinafter, MEAA), methoxyacetic acid having thechemical structure CH₃OCOOH (hereinafter, MAA),(2-methoxy-1-methylethoxy)acetic acid having the chemical structureCH₃OCH₂CH(CH₃)OCH₂COOH (hereinafter, MMEAA), and mixtures thereof. MEAA,MEEAA, and MAA are commercially available from Aldrich Chemical Co., St.Louis, Mo.

The polyether carboxylic acid can be, for example, added to an aqueoussolution of a starting metal salt, and then the resulting mixture can bedried, for example, overnight in an oven at about 120° C. Alternatively,a base (for example, sodium hydroxide) can be added to an aqueoussolution of starting metal salt to form a precipitate (for example, ametal hydroxide), which can be collected (for example, by filtration),washed (for example, in relatively cold water), and dispersed in waterprior to polyether carboxylic acid addition. The resulting mixture canbe reacted by heating, for example, to about 70° C. with overnightstirring. The resulting metal carboxylate salt can be isolated (forexample, by filtration followed by rotary evaporation of the resultingfiltrate) and dried (for example, in a vacuum oven). Other orders andmanners of combination of the starting metal salt and the polyethercarboxylic acid can be utilized. Stoichiometric amounts of the startingmetal salt and the polyether carboxylic acid generally can be used,although a stoichiometric excess of either reactant can be useful.

The above-described metal carboxylate salt(s) can be used in the processof the invention in combination with one or more other salts (forexample, salts such as zinc acetate) having only non-interfering anions(as defined above in reference to Formula I), if desired. Preferably, atleast about 50 mole percent of such a combination, however, will be theabove-described metal carboxylate salt(s). If desired, the other saltshaving non-interfering anions can comprise other metal cations (forexample, at levels up to about 10 mole percent, based upon the totalnumber of moles of metal cation), but preferably all metals in the othersalts are selected from those that form amphoteric metal oxides oroxyhydroxides.

Solvents

Solvents that can be suitable for use in carrying out the process of theinvention include those in which the starting metal carboxylate salts ormetal carboxylate salt precursors and bases can be substantiallysoluble. Such solvents include polar organic solvents (for example,dimethylsulfoxide (DMSO), dimethylformamide (DMF), acetonitrile,alkanols (for example, methanol, ethanol, isopropanol,1-methoxy-2-propanol, and the like, and mixtures thereof),N-methylpyrrolidinone (NMP), water (for example, at temperatures abovethe zinc hydroxide decomposition temperature when using zinc carboxylatestarting salts), and the like, and mixtures thereof.

Preferred solvents can include DMSO, DMF, acetonitrile, NMP, andmixtures thereof (with DMSO being more preferred), due to the relativelyhigh solubility of metal carboxylates in such solvents. Preferredsolvents alternatively can include alkanols (preferably,1-methoxy-2-propanol), however, due to their ease of removal duringpurification. Most preferably, the solvent will be capable of dissolvingthe reactants and products of the process, while keeping the desiredmetal oxide nanoparticles well-dispersed.

Process

The process of the invention can be carried out by combining at leastone base and at least one metal carboxylate salt (preferably, in atleast one solvent). Alternatively, but less preferably, the process canbe carried out by substituting metal carboxylate salt precursors for themetal carboxylate salt. Such precursors can comprise (i) at least onemetal salt comprising a metal cation (selected from metal cations thatform amphoteric metal oxides or oxyhydroxides, as described above) and anon-interfering anion (that is, an anion that is not reactive with thebase, as described above) and (ii) at least one carboxylic acidcomprising from one to four alkyleneoxy moieties (as described above),at least one salt of such a carboxylic acid and a non-interfering,non-metal cation (for example, tetraalkylammonium; preferably,tetramethylammonium), or a mixture thereof (preferably, at least onenon-metal salt). A class of useful metal salts can be represented by thefollowing general formula

M⁺[X⁻]_(n)  (III)

wherein M, X, and n are as defined above for Formula I. A class ofuseful carboxylic acids can be represented by Formula II above. In thisalternative process, an excess amount of base relative to the amountneeded to neutralize all of the carboxylic acid can be utilized to allowreaction of base with the metal present. This alternative processgenerates one mole of water for each mole of acid neutralized. Water inrelatively small amounts can speed the kinetics of growth of ZnOnanoparticles, but the presence of water in relatively larger amountscan cause agglomeration.

Generally, any order and manner of combination of reactants can beutilized, although it can sometimes be preferable to dissolve eachreactant separately in solvent prior to combination. Preferably, asubstoichiometric amount of base relative to the amount of metalcarboxylate salt or metal carboxylate salt precursors (especially whenthe salt is a zinc carboxylate) can be utilized (for example, to ensurethat the resulting metal oxide stays well-dispersed).

Mechanical agitation or stirring can be used, if desired, to facilitatemixing. Optionally, heating can be used to facilitate dissolution,reaction, and/or primary particle size growth. The reactants can becombined in a pressure vessel, if desired (for example, this can beuseful for reactions carried out at temperatures above the boiling pointof a selected solvent).

To influence, for example, the morphology, magnetic properties,conductivity, light absorption or emission characteristics, and/or thecrystallinity of the resulting nanoparticles, various compounds (foreignions) can be added before, during, or after nanoparticle precipitation.Preferred additive compounds include 2nd-4th main group and transitionmetal compounds (more preferably, cobalt, gallium, indium, manganese,magnesium, silicon, and aluminum compounds, and mixtures thereof, mostpreferably, aluminum, gallium, indium, and silicon compounds, andmixtures thereof). Such additive compounds preferably can be added tothe reactant combination in dissolved form and/or preferably can be usedin an amount from about 0.01 to about 10 mole percent, based on thetotal number of moles of metal (present in the form of metalcarboxylate).

The resulting nanoparticles can be isolated (for example, from aresulting sol) and/or purified by using standard techniques such asdecantation (for example, following centrifugation or settlingoptionally induced by cosolvent addition), filtration, rotaryevaporation for solvent removal, dialysis, diafiltration, and the like,and combinations thereof. The characteristics of the resulting productcan be evaluated by ultraviolet-visible spectroscopy (absorptioncharacteristics), X-ray diffraction (crystalline particle size,crystalline phase, and particle size distribution), transmissionelectron microscopy (particle sizes, crystalline phase, and particlesize distributions), and dynamic light scattering (degree ofagglomeration).

The resulting nanoparticles can be used, for example, inorganic-inorganic hybrid materials (for example, for the UV protectionof polymers, paints, coatings, and the like). Preferred embodiments ofthe process of the invention can provide nanoparticles useful in makingUV light absorbing, visible light transparent composites.

EXAMPLES

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention. These examplesare merely for illustrative purposes only and are not meant to belimiting on the scope of the appended claims.

All parts, percentages, ratios, etc. in the examples and the rest of thespecification are by weight, unless noted otherwise. Solvents and otherreagents used were obtained from Sigma-Aldrich Chemical Company, St.Louis, Mo., unless otherwise noted.

Primary Particle Size Determination

The ultraviolet (UV)-visible (Vis) spectrometer used to monitor primaryparticle sizes was a PerkinElmer Lambda 35 instrument (available fromPerkinElmer Life and Analytical Sciences, Wellesley, Mass.) with 1 cmpath length UV-grade quartz sample cells or cuvettes. A small (0.5 g)aliquot of experimental sample was withdrawn from its vial and dilutedwith 24.5 g 200 proof absolute ethanol (USP grade, Aaper Alcohol andChemical Co., Shelbyville, Ky.). The diluted sample was then shaken wellto mix, and approximately 2.5 mL was transferred to a cuvette forUV-visible spectroscopy. The UV-Vis spectrometer was scanned from 500 to280 nm, at a rate of 1920 nm per minute, using a slit width set for 1 nmand a data collection interval of 1 nm. The spectrometer was set tochange from the visible light lamp to the UV light lamp at 326 nm.

The location of the absorbance edge of the sample was determined bydifferentiating the absorbance versus wavelength curve with respect towavelength, using the Savitzsky-Golay procedure supplied with thespectrometer software, where the width parameter was set to 9. A curvewith a distinct peak was the result, the maximum of which was taken tobe the absorbance edge position, λ′_(max). The following equation,described by E. A. Meulenkamp in Journal of Physical Chemistry, B, 102,5556-5572 (1998), was used to determine the particle diameter (d, in nm)from the absorbance edge position (λ′_(max), also in nm).

d={0.017+[(334.56/λ′_(max))−0.8904]^(1/2)}/[(375.64/λ′_(max))−1]

Because light scattering leads to an incorrect measurement of absorbanceand therefore an incorrect determination of primary particle size,primary particle sizes were not calculated when samples scattered to anappreciable amount (for example, due to significant agglomeration). Thisappreciable amount was defined as the absorbance at 400 nm divided bythe absorbance at the top of the absorbance edge. When this number wasgreater than 0.2, no primary particle sizes were calculated.

Degree of Agglomeration

Dynamic light scattering measurements were made using a MalvernNANOSIZER Nano-ZS, Model Number ZEN-3600, particle size analyzer(available from Malvern Instruments, Malvern, U.K.) and were used tomonitor the agglomeration of particles over time. A small (1 g) aliquotwas taken from a sample vial in an oil bath and diluted with 1 gdimethylsulfoxide (DMSO). The diluted sample was mixed well and thentransferred to a glass cuvette. Light scattering data was recorded withthe sample temperature set at 25° C. In transforming the resultingautocorrelation function into particle size, the viscosity (1.98×10⁻³Pa·s; 1.98 cP) and refractive index (1.479) of dimethylsulfoxide wereused. The reported agglomerated particle diameter was based upon anintensity weighted distribution.

Preparation of Zinc 2-(2-Methoxyethoxy)acetate, Zn(MEAc)₂

Zn(MEAc)₂ was prepared following essentially the procedure described byH. M. Xiong et al. in Chemistry of Materials 17, 3062-3064 (2005). To asolution of zinc chloride (27.3 g, 0.2 mole from Mallincrodt Baker,Phillipsburg, N.J.) in water (30 g) was added sodium hydroxide (415 mLof 1.0 N solution in water; Mallincrodt Baker, Phillipsburg, N.J.). Awhite precipitate formed essentially immediately. The resulting mixturewas filtered, and the precipitate was washed with cold water (3 timeswith 500 mL each). The precipitate was then dispersed in water (100 g),and the resulting dispersion was heated to 70° C.2-(2-Methoxyethoxy)acetic acid (51 g, 0.38 mole) was then added to thedispersion, and the resulting mixture was stirred at 70° C. forapproximately 15 hours. The resulting slightly hazy solution was thenfiltered, and the filtrate was concentrated by removing water using arotary evaporator. The resulting solid was dried in a vacuum oven (100°C.) for 15 hours.

Preparation of Zinc 2-[2-(2-Methoxyethoxy)ethoxy]acetate, Zn(MEEAC)₂

Zn(MEEAc)₂ was prepared following essentially the procedure describedabove for Zn(MEAc)₂, except substituting2-[2-(2-methoxyethoxy)ethoxy]acetic acid (66.2 g, 0.37 mole) for2-(2-methoxyethoxy)acetic acid.

Preparation of Zinc Methoxyacetate, Zn(MAC)₂

Zn(MAc)₂ was prepared following essentially the procedure describedabove for Zn(MEAc)₂, except substituting methoxyacetic acid (34.2 g,0.38 mole) for 2-(2-methoxyethoxy)acetic acid.

Preparation of 2-Methoxy-1-methylethoxy Acetic Acid, MMEAA

2-Methoxy-1-methylethoxy acetic acid, MMEAA, was prepared using amodified version of the synthetic method of Example 5 of U.S. Pat. No.3,389,170 (Habicht et al.). A dispersion of sodium hydride (NaH) inmineral oil (60 percent by weight, 48.0 g, 1.2 moles) was weighed into aflame-dried, 3-necked, 1 liter round-bottomed flask, equipped with acondenser, a thermometer, and a rubber septum. Hexanes (100 mL) wereadded to the flask, and the resulting mixture was stirred under nitrogenfor 1 hour. The NaH was allowed to settle, and the hexanes were removedvia cannula. The addition of hexanes, stirring, settling, and removalwere repeated. Anhydrous tetrahydrofuran (hereinafter, THF; 100 mL,DRISOLV grade, EMD Chemicals, Gibbstown, N.J.) was then added to theflask, and the resulting mixture was stirred. The round-bottomed flaskwas then cooled in an ice bath, and the rubber septum was replaced withan addition funnel. Molecular sieve-dried 1-methoxy-2-propanol (270 mL,2.76 moles, Alfa Aesar, Ward Hill, Mass.) was added slowly to the NaH inTHF dispersion. The resulting mixture was allowed to slowly warm to roomtemperature. Chloroacetic acid (47.25 g, 0.5 moles, Alfa Aesar, WardHill, Mass.) was dissolved in anhydrous THF and added to the additionfunnel. This solution was dropped slowly into the round-bottomed flaskso as to maintain a slow reflux. After such addition, the resultingreaction mixture was allowed to reflux for 22 hours.

The solvent (THF and remaining 1-methoxy-2-propanol) was then strippedusing a rotary evaporator. Phosphoric acid (85 percent by weight, 58 g,Mallinckrodt Baker, Phillipsburg, N.J.) was added to the flask, alongwith 300 mL water, resulting in a hazy solution. This solution wasextracted with twelve 150 mL portions of anhydrous diethyl ether(Mallinckrodt Baker, Phillipsburg, N.J.). The ether portions were driedover anhydrous magnesium sulfate powder (EMD Chemicals, Gibbstown,N.J.), filtered, and the ether stripped via rotary evaporations. Thetan-colored portions resulting from the ether extraction were collectedand vacuum distilled (137-140° C./12 Torr), resulting in a cloudy, butcolorless, liquid product. Upon sitting, a small amount of liquid phaseseparated from the main portion of the liquid product. This small amountof liquid phase was removed via pipette and discarded.

The ¹H and ¹³C nuclear magnetic resonance (NMR) spectra of the resultingproduct were consistent with the structure of 2-methoxy-1-methylethoxyacetic acid. Titration of 0.1 M solution of the product in water with0.1 M water solution of sodium hydroxide (Mallinckrodt Baker,Phillipsburg, N.J.) showed a product purity of 99 percent.

Preparation of Zinc 2-Methoxy-1-methylethoxy Acetate, Zn(MMEAc)₂

Zinc 2-methoxy-1-methylethoxy acetate, Zn(MMEAc)₂, was prepared usingthe following modified version of the procedure described by H. M. Xionget al. in Chemistry of Materials 17, 3062-3064 (2005). Zinc chloride(0.05 mole, 27.3 g, Mallinckrodt Baker, Phillipsburg, N.J.) wasdissolved in 30 g water. Sodium hydroxide (100 mL of 1.0 N solution inwater, Mallinckrodt Baker, Phillipsburg, N.J.) was added to theresulting solution. A white precipitate formed immediately. The mixturewas filtered, and the precipitate was washed two times with 200 mL ofcold water. The precipitate was dispersed in 100 g water. The resultingmixture was heated to 70° C., and the above-synthesized2-methoxy-1-methylethoxy acetic acid (0.1025 mole, 15.2 g) was added tothe mixture with magnetic stirring. After 45 minutes of heating, theresulting solution was allowed to cool. Water was removed by rotaryevaporation. The resulting viscous liquid was placed in a vacuum ovenovernight at 100° C.

Examples 1-12 and Comparative Examples 1(a), 1(b), and 2

Stock solutions were prepared as follows: A stock solution of zincacetate (Zn(Ac)₂) (0.0005 mole per g) was made by dissolving dry zincacetate (2.75 g) in dimethyl sulfoxide (27.25 g, OMNISOLV grade, EMDChemicals, Gibbstown, N.J.). A stock solution of Zn(MEAc)₂ (0.0005 moleper g) was prepared by dissolving Zn(MEAc)₂ (4.97 g) in dimethylsulfoxide (25.03 g). A stock solution of Zn(MEEAc)₂ (0.0005 mole per g)was made by dissolving Zn(MEEAc)₂ (5.25 g) in dimethyl sulfoxide (19.75g). A stock solution of Zn(MAc)₂ (0.0005 mole per g) was prepared bydissolving Zn(MAc)₂ (3.65 g) in dimethyl sulfoxide (25.03 g).

Samples containing various amounts of Zn(Ac)₂ and Zn(MEAc)₂ orZn(MEEAc)₂ or Zn(MAc)₂ were then prepared by mixing the amounts (ofstock solution in grams) specified in Table 1 below. Each sample wasplaced in a 40 mL vial, and to each sample was added additional dimethylsulfoxide (20.3 g), with stirring. Each vial was then placed in an oilbath (90° C. for Examples 1-8 and Comparative Examples C1(a) and C1(b);60° C. for Examples 9-12 and Comparative Example C2).Tetramethylammonium hydroxide (2.3 g, 25 weight percent in methanol) wasthen added drop-wise to each sample, and each sample was analyzed byUV-visible spectroscopy and dynamic light scattering at various timeintervals. The results are displayed graphically in FIGS. 1-6.

TABLE 1 Amount Amount of Amount of Amount of of Zn(MEAc)₂ Zn(MEEAc)₂Zn(MAAc)₂ Zn(Ac)₂ Weight Weight Weight Stock Stock Stock Stock PercentPercent Percent Example Solution Solution Solution Solution of of ofNumber (g) (g) (g) (g) Zn(MEAc)₂ Zn(MEEAc)₂ Zn(MAc)₂ C1 (a) 0 0 0 7.4 00 0 1 1.8 0 0 5.6 25 0 0 2 3.7 0 0 3.7 50 0 0 3 5.6 0 0 1.8 75 0 0 4 7.40 0 0 100 0 0 C1 (b) 0 0 0 7.4 0 0 0 5 0 1.8 0 5.6 0 25 0 6 0 3.7 0 3.70 50 0 7 0 5.6 0 1.8 0 75 0 8 0 7.4 0 0 0 100 0 C2 0 0 0 7.4 0 0 0 9 0 01.8 5.6 0 0 25 10  0 0 3.7 3.7 0 0 50 11  0 0 5.6 1.8 0 0 75 12  0 0 7.40 0 0 100

Example 13 ZnO Synthesis Using Zn(MMEAc)₂ in Dimethyl Sulfoxide (DMSO)

Zn(MMEAc)₂ (0.89 g, 2.5 mmole) was weighed into a vial, and 17.56 g DMSOwas added. The resulting solution was heated by placing the vial in anoil bath set at 90° C. Tetramethylammonium hydroxide (25 percent byweight in methanol, 1.55 g, 4.3 mmole) was added to the solution. Growthof zinc oxide primary particles over time was monitored by UV-visiblespectroscopy as described above. Upon cooling completely in DMSO, theresulting particles flocculated. Dilution with a less polar solvent,such as 1-methoxy-2-propanol or ethanol, allowed the particles toredisperse. Consequently, all dynamic light scattering measurements fordetermining agglomeration were carried out by taking a small aliquot ofthe resulting particle-containing reaction mixture and diluting tentimes with 1-methoxy-2-propanol. 1-Methoxy-2-propanol refractive index(1.403) and viscosity values (1.81 cP; 1.81×10⁻³ Pa·s) were then used intransforming the resulting autocorrelation function into particle size.

Example 14 and Comparative Example 3 ZnO Synthesis Using Zn(MMEAc)₂ orZn(Ac)₂ in 1-Methoxy-2-propanol

Zn(MMEAc)₂ (1.33 g, 3.7 mmole, for Example 14) was weighed into a vial,and 26.39 g 1-methoxy-2-propanol was added. The resulting solution washeated by placing the vial in an oil bath set at 80° C.Tetramethylammonium hydroxide (25 percent by weight in methanol, 2.28 g,6.3 mmole) was added to the solution.

Zinc acetate, Zn(Ac)₂, (0.68 g, 3.7 mmole, for Comparative Example 3)was weighed into a vial, and 27.04 g 1-methoxy-2-propanol was added. Theresulting solution was heated by placing the vial in the same oil bathset at 80° C. as above. Tetramethylammonium hydroxide (25 percent byweight in methanol, 2.28 g, 6.3 mmole) was added to the solution.

Growth of zinc oxide primary particles was monitored over time for bothsolutions by UV-visible spectroscopy as described above. The levels ofparticle agglomeration were determined by dilution of small aliquots ofthe resulting particle-containing reaction mixtures in an equal weightof 1-methoxy-2-propanol. Refractive index and viscosity values for1-methoxy-2-propanol were then used as described in Example 13.

The referenced descriptions contained in the patents, patent documents,and publications cited herein are incorporated by reference in theirentirety as if each were individually incorporated. Variousunforeseeable modifications and alterations to this invention willbecome apparent to those skilled in the art without departing from thescope and spirit of this invention. It should be understood that thisinvention is not intended to be unduly limited by the illustrativeembodiments and examples set forth herein and that such examples andembodiments are presented by way of example only, with the scope of theinvention intended to be limited only by the claims set forth herein asfollows:

1. A process comprising (a) combining (1) at least one base and (2) atleast one metal carboxylate salt comprising (i) a metal cation selectedfrom metal cations that form amphoteric metal oxides or oxyhydroxidesand (ii) a carboxylate anion comprising from one to four alkyleneoxymoieties, or metal carboxylate salt precursors comprising (i) at leastone metal salt comprising a said metal cation and a non-interferinganion and (ii) at least one carboxylic acid comprising from one to fouralkyleneoxy moieties, at least one salt of said carboxylic acid and anon-interfering, non-metal cation, or a mixture thereof; and (b)allowing said base and said metal carboxylate salt or said metalcarboxylate salt precursors to react.
 2. The process of claim 1, whereinsaid base is a hydroxyl group-containing base.
 3. The process of claim1, wherein said base is selected from sodium hydroxide, potassiumhydroxide, ammonium hydroxide, tetramethylammonium hydroxide, andmixtures thereof.
 4. The process of claim 1, wherein said metal cationis selected from cations of Be, Ti, V, Mn, Cr, Fe, Co, Ni, Al, Zn, Ga,In, Ge, Sn, Pb, As, Sb, Bi, Te, and Po, and mixtures thereof. 5.(canceled)
 6. The process of claim 1, wherein said metal cation is azinc cation.
 7. The process of claim 1, wherein said carboxylate anionand said carboxylic acid further comprise a terminal alkyl or hydroxylgroup.
 8. The process of claim 1, wherein said carboxylate anion andsaid carboxylic acid further comprise a terminal alkyl group.
 9. Theprocess of claim 8, wherein said alkyl group has from one to four carbonatoms.
 10. The process of claim 1, wherein said carboxylate anion andsaid carboxylic acid further comprise a methyl group.
 11. The process ofclaim 1, wherein said alkyleneoxy moieties are selected frommethyleneoxy, ethyleneoxy, n-propyleneoxy, i-propyleneoxy, butyleneoxy,and the corresponding methoxy- and ethoxy-substituted moieties, andmixtures thereof.
 12. The process of claim 1, wherein said alkyleneoxymoieties comprise moieties selected from ethyleneoxy moieties,propyleneoxy moieties, and mixtures thereof.
 13. The process of claim 1,wherein said alkyleneoxy moieties comprise ethyleneoxy moieties.
 14. Theprocess of claim 13, wherein said ethyleneoxy moieties are one or two innumber.
 15. The process of claim 1, wherein said non-interferingnon-metal cation is tetraalkylammonium, and wherein said non-interferinganion is selected from halide, nitrate, acetate, carbonate, formate,propionate, sulfate, bromate, perchlorate, tribromoacetate,trichloroacetate, trifluoroacetate, and chlorate ions, and mixturesthereof.
 16. (canceled)
 17. The process of claim 1, wherein said metalcarboxylate salt is one of a class that is represented by the followinggeneral formula[R′(OR)_(x)Z(OR)_(w)(CH₂)_(y)COO⁻]_(m)[X⁻]_(n)M^(+(m+n))  (I), saidcarboxylic acid is one of a class that is represented by the followinggeneral formulaR′(OR)_(x)Z(OR)_(w)(CH₂)_(y)COOH  (II), and said metal salt is one of aclass that is represented by the following general formulaM^(+n)[X⁻]  (III), wherein R′ is a linear or branched alkyl group havingfrom 1 to 4 carbon atoms; each R is independently a linear or branchedalkylene moiety having from 1 to 4 carbon atoms; x is an integer of 0 to4; Z is a divalent organic linking moiety; w is an integer of 0 to 4,with the proviso that the sum x+w is an integer of 1 to 4; y is aninteger of 0 to 3; X is a non-interfering anion; m and n are integershaving values such that the sum m+n is equal to the charge of the metalcation, M; and at least 90 mole percent of M (based upon the totalnumber of moles of metal cation) is selected from Be, Ti, V, Mn, Cr, Fe,Co, Ni, Al, Zn, Ga, In, Ge, Sn, Pb, As, Sb, Bi, Te, Po, and mixturesthereof.
 18. The process of claim 17, wherein said Z is a moietynon-directionally selected from the group consisting of a covalent bond,—S—, —C(O)O—, —C═C—, and —C(O)NH—, and combinations thereof.
 19. Theprocess of claim 17, wherein said R′ is methyl, ethyl, or isopropyl;each said R is independently a linear or branched alkylene moiety havingfrom 1 to 3 carbon atoms; said x is an integer of 1 to 3; said Z isselected from the group consisting of a covalent bond, —C(O)O—, —C═C—,—C(O)NH—, and combinations thereof; said w is an integer of 0 to 1, withthe proviso that the sum of x+w is an integer of 1 to 4; said y is aninteger of 0 to 2; said X is an anion selected from halide, nitrate,acetate, carbonate, formate, propionate, sulfate, bromate, perchlorate,tribromoacetate, trichloroacetate, trifluoroacetate, and chlorate ions,and mixtures thereof; and said M is selected from Ti, V, Mn, Cr, Al, Zn,Ga, In, Sn, Pb, and mixtures thereof.
 20. (canceled)
 21. (canceled) 22.The process of claim 1, wherein said metal carboxylate salt is selectedfrom zinc 2-[2-(2-methoxyethoxy)ethoxy]acetate, zinc2-(2-methoxyethoxy)acetate, zinc methoxyacetate, zinc succinatemono-(2-methoxyethyl) ester, zinc succinatemono-[2-(2-methoxyethoxy)ethyl]ester, zinc succinatemono-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}ester, zinc succinatemono-(2-methoxy-1-methylethyl) ester, zinc succinatemono-[2-(2-methoxy-1-methylethoxy)-1-methylethyl]ester, zinc succinatemono-{2-[2-(2-methoxy-1-methylethoxy)-1-methylethoxy]-1-methylethyl}ester,zinc [3-(2-methoxyethoxy)propoxy]acetate, zinc(2-methoxyethoxymethoxy)acetate, zinc succinatemono-{2-[2-(2-methoxyethoxy)-1-methylethoxy]-1-methylethyl}ester, zinc3-[2-(2-methoxyethoxy)ethoxy]propionate, zinc[2-(2-methoxyethoxy)-1-methylethoxy]acetate, zinc pentanedioatemono-[2-(2-methoxyethoxy)ethyl]ester, zinc pentanedioatemono-(2-methoxy-1-methylethyl) ester, zinc(2-methoxyethoxycarbonylmethoxy)acetate, zinc(2-methoxy-1-methylethoxycarbonylmethoxy)acetate, zinc but-2-enedioatemono-(2-methoxyethyl) ester, zinc N-(2-methoxyethyl)succinamate, zinc[(2-methoxyethylcarbamoyl)methoxy]acetate, zinc[2-(2-ethoxyethoxy)ethoxy]acetate, zinc[2-(2-isopropoxyethoxy)ethoxy]acetate, zinc{2-[2-(2-tert-butoxyethoxy)ethoxy]ethoxy}acetate, zinc{2-[2-(2-sec-butoxyethoxy)ethoxy]ethoxy}acetate, zinc2,3-dimethoxypropionate, zinc succinate mono-(2,3-dimethoxypropyl)ester, zinc succinate mono-(2-methoxy-1-methoxymethylethyl) ester, zinc3-(2-methoxyethylcarbamoyl)acetate, zinc (2-hydroxyethoxy)acetate, zinc[2-(2-hydroxyethoxy)ethoxy]acetate, zinc3-{2-[2-(2-methoxy-1-methylethoxy)-1-methylethoxy]-1-methylethoxy}propionate,zinc 3-(2-methoxy-1-methylethoxy)propionate, zinc{2-[2-(2-methoxy-1-methylethoxy)-1-methylethoxy]-1-methylethoxy}acetate,zinc (2-methoxy-1-methylethoxy)acetate, and mixtures thereof.
 23. Theprocess of claim 1, wherein said metal carboxylate salt is used incombination with at least one other salt, said other salt having onlynon-interfering anions.
 24. A process comprising (a) combining (1) atleast one hydroxyl-group containing base and (2) at least one metalcarboxylate salt selected from zinc2-[2-(2-methoxyethoxy)ethoxy]acetate, zinc 2-(2-methoxyethoxy)acetate,zinc methoxyacetate, zinc (2-methoxy-1-methylethoxy)acetate, andmixtures thereof; and (b) allowing said base and said salt to react. 25.(canceled)